20x is a lot of zoom for the money. Canon does make a 100x zoom lens but I imagine it's $1K or more per x. To be a bit more serious tele extenders are usually a disaster. I'm guessing that you really want more magnification and not more zoom ratio in which case still camera lenses with modest zoom ratios may serve. There is an adapter for the Canon EOS lenses and third party vendors make adapters for other makes. Remember the 7.2 magnification factor from the small sensor size (i.e. a 100 mm lens yields 35mm equivalent focal length of 720 mm).

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